Conventionally, uninterruptible power supply apparatuses have been widely used as power supply apparatuses for stably supplying alternating current (AC) power to an important load such as a computer system. In normal operation, an uninterruptible power supply apparatus converts commercial AC power into direct current (DC) power and supplies the DC power to a battery and also converts the DC power into AC power with commercial frequency to supply the AC power to a load. In power failure, the uninterruptible power supply apparatus converts DC power of the battery into AC power with commercial frequency to supply the AC power to the load. As such, even in power failure, AC power with commercial frequency can be supplied to the load.
There also is an uninterruptible power supply system including a plurality of uninterruptible power supply apparatuses connected in parallel. In this uninterruptible power supply system, even if one of the uninterruptible power supply apparatuses fails, the rest of the uninterruptible power supply apparatuses can drive the load (for example, see Patent Literature 1).